I also, honestly, feel that spell-check and word-prediction have helped my non-2e child improve her spelling tremendously.

Do you think so? I'm not trying to be snarky. It just seems like the skill of spelling is in perpetual decline, which I suppose I would partly attribute to spell-check and word prediction. Why learn to spell if the machine will do it for you?

I wonder if anyone has studied this.

BTW, I don't think spelling is the world's most important skill. Being able to write clearly (which is also a skill that is in decline) is far more important. I can definitely see how being able to dictate could help 2E students enormously with this.